Then in 2015, a Confederate-sympathizing white supremacist, Dylann S. Roof, murdered nine black churchgoers in Charleston, S.C. In a dramatic reaction, South Carolina removed the Confederate battle flag from the State House grounds altogether, weeks after Alabama’s governor ordered four flags removed outside the State Capitol in Montgomery. Calls went up for the removal of other relics of Confederate memory, including the battle flag’s place in the corner of the Mississippi state flag, and major retailers, including Walmart, stopped selling merchandise that included the battle flag.

The backlash was swift. Membership in the Sons of Confederate Veterans spiked, and large pro-flag demonstrations crowded the grounds of Southern state capitols. The rallying cry was still mostly focused on heritage, but a new and more incendiary faction had begun to emerge.

Starting in the early 2010s, a growing number of young, openly white nationalist Southerners began joining Southern nationalist organizations, primarily the Alabama-based League of the South. Mr. Griffin, an Auburn University graduate who had become radicalized by books and in online forums that had little to do with the Confederacy, was among them.